Where a lower local noise standard is adopted outside of the Part process, 49 USC requires that the land use compatibility plan be developed cooperatively by the airport sponsor and local jurisdiction to be eligible for a grant. Addi- tional information on these requirements is addressed in Paragraph b. Sound has proper- ties of both fluids and waves. It propagates outward from its source at high speed, bends around interposing structures, is partially reflected and partially absorbed by incident surfaces, and radi- ates through structures, which attenuate i.
Improved interior NLR the difference in sound level from exterior to interior is the objective of SIPs, and this NLR is achieved by retrofitting structures with building elements having higher sound transmission loss TL properties. Acoustical Engineering 53 Three aspects of noise are important in determining subjective response: 1. Level i.
The frequency composition or spectrum of the sound. The variation in sound level with time. Sound levels are measured and expressed in decibels, with 0 dB roughly equal to that level at the threshold of hearing. Sound is a measure of the pressure fluctuations per second, measured in units of hertz Hz. Most sounds do not consist of a single frequency but are composed of a broad band of frequencies differing in level. The characterization of sound level magnitude with respect to frequency is the sound spectrum.
Figure 4. Changes in sound level and combinations of sound levels are nonlinear and do not behave as most other physical phenomena. Because the level and frequency of sound are perceived in a nonlinear way, the decibel scale is used to describe sound levels; the frequency scale is also mea- sured in logarithmic increments. Decibels, measuring sound energy, combine logarithmically.
Section 10.2 – Nomenclature
Courtesy of Charles M. Salter Associates, Inc. Range of sound spectra. It would take 10 identical cars passing by simultaneously to be judged as twice as loud as the single car pass-by, though this would be a tenfold, or dB, increase in sound level. The rules and examples for decibel addition used in community noise prediction are given in Table 4.
Interference with activities such as speech, sleep, and learning. Physiological effects such as anxiety or hearing loss.
Subjective effects of annoyance, nuisance, and dissatisfaction. No universal measure for the subjective effects of noise has been developed, nor does a mea- sure exist for human reactions from noise annoyance. This is primarily due to the wide variation of individual attitude regarding noise sources. For aircraft noise, typical reactions vary from annoyance to anxiety to fear. In general, the more a new noise exceeds the prior noise, the less acceptable it is. Therefore, a new noise source will be judged more annoying in a quiet area than it would be in a noisier location.
There are two types of noise impact: 1. Absolute impacts, whereby noise level or noise exposure exceeds a specified numerical stan- dard, and 2. Relative impacts, whereby noise level or noise exposure increases by a specified value. Changes in the noise environment cause a relative impact; the magnitude of a noise environ- ment causes an absolute impact.
Most people acclimate somewhat to their noise environment. The simplest method, A-weighting, is generally used so that measurements can be made and noise impacts readily assessed using basic acoustical instrumentation.
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This method evaluates audible frequencies by using a single weighting filter that progressively de-emphasizes frequency components below Hz and above Hz. This frequency bias reflects the relative decreased human sensitivity to low frequencies and to extreme high frequencies.
A-weighting is applied by an electrical filter in all U. Decibel addition used in community noise prediction.
Acoustical Engineering 55 4. Although a single sound level may adequately describe the noise at any instant in time, airport and other community noise levels vary continuously. Most community noise is produced by many noise sources, which create a relatively steady background noise that has no identifiable source. These sources change gradually throughout the day and include traf- fic, wind through foliage, and distant industrial activities. Superimposed on this slowly varying background is a succession of identifiable noise events of brief duration.follow
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These include nearby activities, such as single vehicle pass-bys or aircraft flyovers, which cause the community noise level to vary from instant to instant. This fluctuating series of noise levels combines to form the noise exposure profile of a community. For purposes of quantifying noise that varies over a period of time, a standard term, equivalent sound level, has been adopted in the United States and internationally.
Equivalent sound level is an energy average that takes varying sound levels of a time period and describes them as one constant noise level i. It is a construction of that constant sound level containing the same acoustic energy as the varying sound level during the same time period. Discrete, short-duration transient noise events, such as aircraft flyovers, may be described by their maximum A-weighted noise level or by their sound exposure level SEL.
A-weighted network. Maximum levels of transient events vary with instantaneous propagation, measurement system time constant, and receiver conditions, while a total energy measure, like SEL, is more stable. The SEL of a transient event is a measure of the acoustic energy normalized to a constant duration of 1 second. The SEL dif- fers from the Leq in that SEL is the constant sound level containing the same acoustic energy as a 1-second event, whereas the Leq is the constant sound level containing the same acoustic energy over the entire measurement period.
SEL values may be summed on an energy basis to compute Leq values over any period of time.
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This is useful for modeling noise in areas exposed to numerous transient noise events, such as communities around airports. Hourly Leq values are called hourly noise levels HNLs.
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In determining the daily measure of community noise, it is important to account for the difference in human response to daytime and nighttime noise. During the night, people are more often at home and exterior background noise levels are generally lower than during the day, which causes exterior noise intrusions to become more noticeable.
For these reasons, most people are more sensitive to noise at night than during the day.
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The DNL represents the hour, A-weighted equivalent sound level with a dB penalty added for nighttime noise between p. Sound exposure level.
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Acoustical Engineering 57 evening i. In order to achieve conformity in efforts across the country in varying climates and construction typologies, that work has to meet a consistent design standard. A target post-retrofit interior DNL of 45 dB in all habitable rooms. A minimum NLR improvement of 5 dB.
A series of perception tests11 revealed that changes in noise exposure were per- ceived as follows: 1. Except under special conditions, a change in sound level of 1 dB cannot be perceived. Outside of the laboratory, a 3-dB change is considered to be a just-noticeable difference.
Hourly noise levels and annual metrics. Zwicker and B. An increase or decrease in level of at least 5 dB is required before any noticeable change in response would be expected. A dB increase is subjectively heard as an approximate doubling in loudness. The changes in perceived airport noise exposure, however, are considerably more complex for two reasons: 1. The spectra of individual aircraft flyovers change substantially between individual events, so the comparison in loudness is complicated by comparing dissimilar sounds.
Issues of annoy- ance, nuisance, dissatisfaction, speech interference, sleep interference, learning impairment, anxiety, and hearing loss all affect subjective response to changes in noise level. More significantly, noise exposure is an integrated measure of sounds over a period of time, whereas sound level is simpler and immediate. Subjective comparison of any sensory values is greatly affected by latency, or the time between events.
So comparisons of long-term noise exposure measures, such as DNL, will yield much more varied responses than will the imme- diate changes in noise level used for this perception test. The FAA, in establishing performance criteria for SIPs, ensured that sound insulation pro- gram treatments would provide an audible improvement in affected buildings. Design approaches include: 1. Apply a uniform noise reduction treatment standard to create a homogeneous building envelope using consistent treatments.
Program sponsors and consultants are advised to consult with their local ADO for fur- ther clarification regarding this issue. In the first approach, each room could have a different existing NLR depending on the ratio and composition of building materials. Achieving a uniform minimum 5-dB treatment could require different treatments for each room to achieve the required NLR. The second approach allows for use of uniform building materials and construction proce- dures throughout the program.
This provides considerable cost savings in both material and labor. Acoustical Engineering 59 However, each room receives a slightly different NLR improvement, but each room receives a similar interior noise environment after retrofit. In the third approach, the exterior envelope of the whole building is reviewed for consistency of construction and building elements, and then individual rooms are verified for specific per- formance issues.
This allows for use of uniform building materials and construction procedures for the majority of the treatments and acknowledges that more retrofit may be needed in limited cases to provide a continuous STC 40 building envelope. The majority of residential SIPs employ some form of the second or third approaches for treatment design; few programs attempt to achieve specific NLR performance for each room. Consequently, when applying a uniform sound transmission class STC performance enve- lope across a program, older homes and those having poorer pre-retrofit NLR performance will realize a greater NLR improvement on average 7 dB to 8 dB than newer and well-maintained homes, which may only realize a 4-dB to 5-dB improvement from the same treatments.
In addition to the NLR properties of basic building elements, another significant noise path is the presence of acoustical leaks, termed flanking paths. These are typically cracks or poor seals where air and sound may infiltrate. Flanking may significantly degrade sound insulation perfor- mance and requires treatment in every instance. Sound has the property of always infiltrating the weakest spot. It is not feasible to apply excessive acoustical treatment in one location while allowing for flanking in another; therefore, attention will need to be paid to the building enve- lope beyond the major fenestration openings.
Given differences in room exposure, there has been considerable discussion as to how the minimum 5-dB NLR improvement criterion is to be applied in sound insulation programs. Alternative interpretations include: 1.
Related Chapter 004, Material Balances
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